Initial High Dynamic Range Imaging experiments

The images in this gallery are my initial experiments with photographic (as compared to CG) High Dynamic Range imaging and were created during 2008 using a unique way of seeing with the camera. High Dynamic Range Imaging (HDRI) is a technique that allows a greater dynamic range of exposures (the value between light and dark images) than normal imaging techniques. Each final HDRI image is composed of five unique images of the same scene, with the shutter speed altered in each one to effectively bracket (in this case from 3 stops under to 3 stops over the median optimal exposure) for the image. When combined, the images are no longer bound by the conventional limitations of photography due to their extra 6-stop latitude, pushing the dynamic range of the scene from what the sensor in the camera can output (in the D300's case about 8 stops, or EVs) to something way beyond that (in this case about 13-14 stops).

This technique is used because all of the scenes in this gallery have variations in radiance that are much greater than that which could be recorded by the camera. Each of the bracketed images contains a range of radiance that is limited, so not all of the scene details can be displayed by any one image. An algorithm is applied to the five images to recreate the high dynamic range radiance "map" of the original scene. The HDRI image is then processed by a local tone mapping algorithm which applies the "anchoring theory of lightness perception" to the entire image, thereby remapping the HDRI image into conventional image space while maintaining an approximation of the HDRI image. The tonemapping algorithms used look for local changes in contrast and then emphasize them, which is akin to a type of sharpening... but sharpening in the realm of the dynamic range/radiance instead of normal sharpening spatially, which is simply a kind of local contrast enhancement. The concepts behind HDRI imaging were originally developed by Charles Wycoff in the 1940s for a series of images of nuclear explosions that he created for the cover of Life Magazine. Greg Ward resurrected this process in the late '80s in order to develop the process of Image Based Lighting for the film visual effects industry (allowing the compositing of synthetic imagery into an arbitrary scene by first capturing an HDR panorama of that scene on the set and then using that radiance image as a lighting source), thereby matching the original real-life scene exactly with the 3D synthetic environment. The process of tone mapping together with bracketed exposures of normal images shot with DSLRs (the technique used in this book) was first published in 1995 by Steve Mann in a research article entitled "On being 'Undigital' with Digital Cameras: Extending Dynamic Range by Combining Differently Exposed Pictures".

At first glance most people perceive the images as being highly saturated, but what you are truly seeing is the novel saturation in the highlights and shadows which normally would be beyond the range of conventional imaging techniques. The human eye has a 100,000:1 contrast ratio, and using this technique we can manipulate the fixed 1000:1 contrast ratio of a photographic image into approximating the radiance resolving power of the human eye. In fact, a printed image may only have a contrast ratio of 200:1 or less, making the final result quite astounding, considering the limitations of the printed medium.

In my own experience, the images capture the fullness of life evident on Mt. Tamalpais that I experience on my walks in a way that conventional photography has never been able to. Using this technique, subtleties in the color of a scene that may not even be evident in the original scene tend to pop out, particularly within the gradations of the sky. The images oftentimes evoke the feeling of viewing one of the old hand-painted postcards from before the days of color film. The technique is far from perfect and has its share of artifacts (wind being the chief culprit, along with chromatic aberration), but even still, it evokes strong emotions and memories. Over the past 3 years I've learned how to layer the tonemapped output with original median exposures in photoshop, and by using masking, produce a much more subtle and natural result.